another baby?

Last night me and my SO were just relaxing and said he wanted to be intimate. I said only if you stay in me(we joke all the time no force or anything) well he said ok. And I said really? We already have an 8 month old. And he said yea and I said we can't have another one yet... maybe when he's five. And he said he was already 5(yea 5 months) I'm in my fertile week and I'm afraid that if we were to do this and there is two lines he'll get nervous. He's a really great dad and we can afford another one. What would you do?